1. Field of the Invention
The present invention relates generally to noise reduction circuits, and more particularly to noise reduction circuits for AC power, flexible noise reduction circuits for DC power supplies, and noise reduction circuits for AC power neutral lines.
2. Discussion of Related Art Including Information Disclosed Under 37 CFR §§1.97, 1.98:
Alternating current power supplied by the utilities power grid is contaminated with noise from a wide variety of sources, including electric appliances, computers, equipment having switching power supplies, automotive ignition systems, radio transmitters, etc. These and other sources of noise and contamination leave the power spectrum of AC power dense with harmonic and spurious noise. When this noise is left untreated, it enters sensitive electronic equipment and degrades performance in a number of ways. Specifically, audio and video equipment produce less pleasing results for the end user. Laboratory equipment, imaging equipment, and other commercial and scientific equipment suffer as well.
Prior solutions to the problem of noise in AC power circuits fit into three main categories: passive power conditioners, power regenerators, and power correctors. The invention fits into the category of power correctors, but for the sake of background, a brief statement for each of the categories follows.
In countries where AC power is supplied by two wires, one of which is electrically tied to a ground connection (earth), the power is said to be single ended or unbalanced. The United States is one such country. The line that is tied to ground is called the neutral line. For optimum performance of electrical equipment, it is desired that the noise level on the neutral line be suppressed with respect to ground. Though the neutral line is connected to ground at the entrance to a building, the neutral line is not usually electrically quiet at the service jack (wall) because of voltage drops that occur along the length of the neutral wire in response to current provided by the neutral line. This neutral wire noise is the subject of the second preferred embodiment of the present invention disclosure.
Prior solutions to the problem of noise in the neutral line of AC power fit into two main categories: passive filtering and balanced power regeneration.
Passive power conditioners that address either or both of the aforementioned problems take the form of low pass or band pass filters. They are generally capable of some degree of effectiveness, particularly at frequencies above 100 KHz. In order to address noise contamination close in frequency to the 50-60 Hz power grid main, the filter components must be quite large and expensive. For this reason, solutions in this category that are price competitive for consumer use are only marginally effective.
In the case of the neutral line for AC power, the problem is made more difficult because of the need to avoid a ground fault. A ground fault is a condition in which the ground wire is carrying significant current. This condition is not acceptable by any existing electrical codes. One cannot therefore simply connect ground and neutral together, or tie them together through a large capacitor. For this reason, solutions in this category that are price competitive for consumer use are only marginally effective.
Power regenerators operate as one or more high voltage-high current amplifiers which amplify a 50-60 Hz signal source to the voltage of the utility power. When used in connection with neutral lines for AC power, these regenerators are often configured to produce balanced power. In balanced power the incoming AC is regenerated in split-voltage fashion, where half of the voltage is presented to the hot line and half to the neutral line. The neutral line is therefore treated by this approach.
In both cases—AC power and neutral lines for AC power—regenerators suffer from a low degree of energy efficiency typical of amplifiers, less than 50%. The problem is further compounded by the high degree of linearity that is required from the amplifier in order to avoid the regeneration of harmonic distortion. More sophisticated modes of amplifier operation are able to permit increased efficiency, but with exacerbated complexity and cost. While products in this category tend to be relatively effective, they are also known to be inefficient, heat producing, large, heavy, and expensive.
Power correctors operate by adding or subtracting a small voltage to the incoming AC voltage, to the right degree at each instant in time to correct the incoming voltage to a sine wave. Stated alternatively, the incoming voltage is represented as an error voltage added to a perfect sine wave. Power correctors evaluate the incoming AC, determine how much of the voltage is error, and subtract that error from the incoming voltage. The end result is an output waveform that is sinusoidal, devoid of the incoming spectral impurities. Power correctors have a fundamental advantage over regenerators in that they are not required to process the entire voltage waveform, only the error. For this reason, the amplifiers) involved operates on much lower voltage and the efficiency is proportionally increased.
In the case of AC power, practical realizations of power correctors have suffered from several limiting constraints centering on methods of subtracting the error from the incoming voltage. In one prior art solution, the method of subtracting the error involves the use of an isolation transformer that handles the entire AC power, resulting in a heavy and costly apparatus. In another solution, the error is subtracted through a small transformer in the AC current path. Such an approach has limited effectiveness because of the finite bandwidth of the transformer.
In the case of neutral lines for AC power, to the knowledge of the present inventor, power correctors have not previously included treatment of the neutral line for noise reduction by means other than the passive techniques described and listed above, except in the case of balanced power correctors.
In addition to the noise problems inherent in AC power lines and AC power neutral lines, direct current power supplies are plagued with noise from various sources. First, there is the noise coming in on the AC power grid, as described above. Second, there is the rectifier switching noise. Third, there is noise from the filler capacitors, transformer, and other passive components. Fourth, there is noise generated by active regulator circuits. Fifth, there is noise generated by the load circuit. There are no doubt other sources of noise.
Typically, power supply circuits treat these sources of noise with large filter capacitors aimed at “shorting out” the noise by bypassing it to ground. The effectiveness of this approach is quite limited because the finite effective series series resistance of the filter capacitors in combination with the relatively low source resistance of the power supply limits the possible noise attenuation. Sometimes a high quality film capacitor is placed in parallel with the electrolytic filler capacitor to improve (reduce) series resistance at high frequencies. While this certainly helps, it does not mitigate the underlying problem because sufficiently large capacitance values are impractical.
Other attempts to address the noise problem involve active voltage regulation circuits. Often this approach takes the form of inexpensive three-terminal integrated circuit regulators common in the industry. This approach improves noise levels in the lower part of the spectrum by compressing the noise against a voltage reference, such as a zener diode. The resultant reference voltage is then scaled and amplified by an operational amplifier and presented to the load. Unfortunately, the voltage reference produces significant unwanted noise. To make matters worse, the operational amplifier is normally setup to produce gain greater than unity so that it can produce the desired output voltage from a convenient value voltage reference, and this means that the reference noise is amplified before being presented to the load.
Common to many of the prior art solutions is a custom engineered approach which forbids implementation after the fad, but instead which requires power supply re-design in order to upgrade.
The prior art devices and circuits reflect the current state of the art of which the present inventor is aware. Reference to, and discussion of, this prior art is intended to aid in discharging Applicant's acknowledged duty of candor in disclosing information that may be relevant to the examination of claims to the present invention. However, it is respectfully submitted that none of the above-indicated prior art discloses, teaches, suggests, shows, or otherwise renders obvious, either singly or when considered in combination, the invention described and claimed herein.